Lithographic apparatus with alignment subsystem, device manufacturing method, and device manufactured thereby

ABSTRACT

A lithographic apparatus according to one embodiment of the invention includes an alignment subsystem configured to align the substrate on the substrate table relative to the patterning structure. The alignment structure comprises a non-periodic feature which may be detectable as e.g. a capture position or a check position using a reference grating in the alignment subsystem. The non-periodic feature may cause a phase effect in the detected signal of the alignment subsystem or an amplitude effect.

RELATED APPLICATIONS

This application claims benefit of and is a divisional application ofU.S. patent application Ser. No. 10/736,230, filed Dec. 16, 2003, whichclaims priority from European Patent Application EP 02080334.2, filedDec. 16, 2002; European Patent Application EP 03075432.9, filed Feb. 14,2003; and European Patent Application EP 03076010.2, filed Apr. 4, 2003,all of these documents being incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to lithographic projection apparatus andmethods.

BACKGROUND

The term “patterning structure” as here employed should be broadlyinterpreted as referring to any structure or field that may be used toendow an incoming radiation beam with a patterned cross-section,corresponding to a pattern that is to be created in a target portion ofa substrate; the term “light valve” can also be used in this context. Itshould be appreciated that the pattern “displayed” on the patterningstructure may differ substantially from the pattern eventuallytransferred to e.g. a substrate or layer thereof (e.g. where pre-biasingof features, optical proximity correction features, phase and/orpolarization variation techniques, and/or multiple exposure techniquesare used). Generally, such a pattern will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit or other device (see below). A patterningstructure may be reflective and/or transmissive. Examples of patterningstructure include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the undiffracted light can        be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An array of grating light valves        (GLVs) may also be used in a corresponding manner, where each        GLV may include a plurality of reflective ribbons that can be        deformed relative to one another (e.g. by application of an        electric potential) to form a grating that reflects incident        light as diffracted light. A further alternative embodiment of a        programmable mirror array employs a matrix arrangement of very        small (possibly microscopic) mirrors, each of which can be        individually tilted about an axis by applying a suitable        localized electric field, or by employing piezoelectric        actuation means. For example, the mirrors may be        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning structure can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat. No.        5,296,891 and U.S. Pat. No. 5,523,193 and PCT patent        applications WO 98/38597 and WO 98/33096, which documents are        incorporated herein by reference. In the case of a programmable        mirror array, the support structure may be embodied as a frame        or table, for example, which may be fixed or movable as        required.    -   A programmable LCD panel. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask (or“reticle”) and mask table (or “reticle table”); however, the generalprinciples discussed in such instances should be seen in the broadercontext of the patterning structure as hereabove set forth.

A lithographic apparatus may be used to apply a desired pattern onto asurface (e.g. a target portion of a substrate). Lithographic projectionapparatus can be used, for example, in the manufacture of integratedcircuits (ICs). In such a case, the patterning structure may generate acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g. comprising one or moredies and/or portion(s) thereof) on a substrate (e.g. a wafer of siliconor other semiconductor material) that has been coated with a layer ofradiation-sensitive material (e.g. resist). In general, a single waferwill contain a whole matrix or network of adjacent target portions thatare successively irradiated via the projection system (e.g. one at atime).

Among current apparatus that employ patterning by a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Aprojection beam in a scanning type of apparatus may have the form of aslit with a slit width in the scanning direction. More information withregard to lithographic devices as here described can be gleaned, forexample, from U.S. Pat. No. 6,046,792, which is incorporated herein byreference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (e.g.resist). Prior to this imaging procedure, the substrate may undergovarious other procedures such as priming, resist coating, and/or a softbake. After exposure, the substrate may be subjected to other proceduressuch as a post-exposure bake (PEB), development, a hard bake, and/ormeasurement/inspection of the imaged features. This set of proceduresmay be used as a basis to pattern an individual layer of a device (e.g.an IC). For example, these transfer procedures may result in a patternedlayer of resist on the substrate. One or more pattern processes mayfollow, such as deposition, etching, ion-implantation (doping),metallization, oxidation, chemo-mechanical polishing, etc., all of whichmay be intended to create, modify, or finish an individual layer. Ifseveral layers are required, then the whole procedure, or a variantthereof, may be repeated for each new layer. Eventually, an array ofdevices will be present on the substrate (wafer). These devices are thenseparated from one another by a technique such as dicing or sawing,whence the individual devices can be mounted on a carrier, connected topins, etc. Further information regarding such processes can be obtained,for example, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4.

A substrate as referred to herein may be processed before or afterexposure: for example, in a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once (for example, in order tocreate a multi-layer IC), so that the term substrate as used herein mayalso refer to a substrate that already contains multiple processedlayers.

The term “projection system” should be broadly interpreted asencompassing various types of projection system, including refractiveoptics, reflective optics, and catadioptric systems, for example. Aparticular projection system may be selected based on factors such as atype of exposure radiation used, any immersion fluid(s) or gas-filledareas in the exposure path, whether a vacuum is used in all or part ofthe exposure path, etc. For the sake of simplicity, the projectionsystem may hereinafter be referred to as the “lens.” The radiationsystem may also include components operating according to any of thesedesign types for directing, shaping, reducing, enlarging, patterning,and/or otherwise controlling the projection beam of radiation, and suchcomponents may also be referred to below, collectively or singularly, asa “lens.”

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTApplication No. WO 98/40791, which documents are incorporated herein byreference.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.water) so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. The use of immersiontechniques to increase the effective numerical aperture of projectionsystems is well known in the art.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams (such as ion or electronbeams).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, DNA analysis. devices, etc. The skilled artisan will appreciatethat, in the context of such alternative applications, any use of theterms “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “substrate” and “target portion”,respectively.

A phase grating alignment technique may have the problem that it onlyworks when sufficiently large alignment structures and/or referencestructures can be used such that the substrate can be initiallypositioned to image the alignment structure onto the referencestructure. In other words, such a technique may require the capturerange of the phase grating alignment to be sufficiently large. Largealignment structures and/or reference structures are expensive, e.g.because they occupy space on the substrate which could otherwise containcircuit components. Therefore, it may be desirable to reduce the area ofthe alignment structures and/or the reference structures.

SUMMARY

A device manufacturing method according to an embodiment of theinvention includes using a patterning structure to pattern a beam ofradiation according to a desired pattern and aligning a substrate,including an alignment structure having spatially varying opticalproperties, relative to the patterning structure. In this method,aligning a substrate includes processing light affected by the alignmentstructure to produce measurement light of which the intensity varieswith the relative position of (1) the spatially periodic alignmentstructure and (2) a reference position relating to the patterningstructure. Aligning a substrate also includes measuring at least one ofintensity information and phase information of the measurement light andcontrolling a relative position of the substrate and the patterningstructure based on the measured information, and detecting a position ofa non-periodic feature of the alignment structure.

An alignment structure according to an embodiment of the inventionincludes at least one phase grating mark having a plurality of adjacentlines and spaces with a predetermined periodicity. The alignmentstructure comprises a non-periodic feature located between two parts ofthe alignment structure that have predetermined periodicities along aline. Substrates and lithographic apparatus according to embodiments ofthe invention are also disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts an alignment subsystem of the lithographic apparatus ofFIG. 1;

FIG. 3 depicts an alignment structure according to an embodiment of thepresent invention;

FIGS. 4 a and 4 b depict an alignment subsystem detection signal for twoimplementations of the alignment structure of FIG. 3;

FIG. 5 depicts an alignment structure according to another embodiment ofthe present invention, in combination with the detected phase signal ofthe alignment subsystem;

FIG. 6 depicts a position-dependent period change and resulting phaseprofile of an alignment structure according to a further embodiment ofthe present invention;

FIG. 7 depicts schematically an alignment subsystem of a lithographicapparatus according to a further embodiment of the present invention;

FIG. 8 depicts a detected signal of the alignment subsystem of FIG. 7;

FIG. 9 depicts an alignment procedure for a wafer using the embodimentof FIG. 7;

FIG. 10 depicts an alignment structure according to a further embodimentof the present invention;

FIG. 11 depicts a flow-chart for a method of aligning a substrate and amask pattern according to an embodiment of the invention;

FIG. 12 depicts a top view of part of a substrate;

FIG. 13 depicts a detection signal;

FIG. 14 depicts a further top view of part of a substrate; and

FIG. 15 depicts an alternative alignment subsystem.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

Embodiments of the invention include, for example, methods and apparatusthat may be used to provide for alignment of a substrate and apatterning structure in which a capture range or a robustness of aperiodic alignment system is improved.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

A radiation system configured to supply (e.g. having structure capableof supplying) a projection beam of radiation. In this particularexample, the radiation system Ex, IL, for supplying a projection beam PBof radiation (e.g. UV or EUV radiation) also comprises a radiationsource LA;

A support structure configured to support a patterning structure capableof patterning the projection beam. In this example, a first object table(mask table) MT is provided with a mask holder for holding a mask MA(e.g. a reticle), and is connected to a first positioning structure foraccurately positioning the mask with respect to item PL;

A second object table (substrate table) configured to hold a substrate.In this example, substrate table WT is provided with a substrate holderfor holding a substrate W (e.g. a resist-coated semiconductor wafer),and is connected to a second positioning structure for accuratelypositioning the substrate with respect to item PL and (e.g.interferometric) measurement structure IF, which is configured toaccurately indicate the position of the substrate and/or substrate tablewith respect to lens PL; and

A projection system (“lens”) configured to project the patterned beam.In this example, projection system PL (e.g. a refractive lens group, acatadioptric or catoptric system, and/or a mirror system) is configuredto image an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies and/or portion(s) thereof) of thesubstrate W. Alternatively, the projection system may project images ofsecondary sources for which the elements of a programmable patterningstructure may act as shutters. The projection system may also include amicrolens array (MLA), e.g. to form the secondary sources and to projectmicrospots onto the substrate.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning structure, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. a mercury lamp, an excimer laser, an electron gun, alaser-produced plasma source or discharge plasma source, or an undulatorprovided around the path of an electron beam in a storage ring orsynchrotron) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed a conditioning structure or field, such as a beam expander Ex,for example. The illuminator IL may comprise an adjusting structure orfield AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam, which may affect the angular distribution ofthe radiation energy delivered by the projection beam at, for example,the substrate. In addition, the apparatus will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable direction mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed (alternatively, having been selectivelyreflected by) the mask MA, the beam PB passes through the lens PL, whichfocuses the beam PB onto a target portion C of the substrate W. With theaid of the second positioning structure (and interferometric measuringstructure IF), the substrate table WT can be moved accurately, e.g. soas to position different target portions C in the path of the beam PB.Similarly, the first positioning structure can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. Mask MA and substrate W may bealigned using mask alignment marks M1, M2 and substrate alignment marksP1, P2.

The depicted apparatus can be used in several different modes:

-   -   1. In step mode, the mask table MT is kept essentially        stationary, and an entire mask image is projected at once (i.e.        in a single “flash”) onto a target portion C. The substrate        table WT is then shifted in the x and/or y directions so that a        different target portion C can be irradiated by the beam PB;    -   2. In scan mode, essentially the same scenario applies, except        that a given target portion C is not exposed in a single        “flash”. Instead, the mask table MT is movable in a given        direction (the so-called “scan direction”, e.g. the y direction)        with a speed v, so that the projection beam PB is caused to scan        over a mask image. Concurrently, the substrate table WT is        simultaneously moved in the same or opposite direction at a        speed V=Mv, in which M is the magnification of the lens PL        (typically, M=¼ or ⅕). In this manner, a relatively large target        portion C can be exposed, without having to compromise on        resolution.    -   3. In another mode, the mask table MT is kept essentially        stationary holding a programmable patterning structure, and the        substrate table WT is moved or scanned while a pattern imparted        to the projection beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning structure is updated as required        after each movement of the substrate table WT or in between        successive radiation pulses during a scan. This mode of        operation can be readily applied to maskless lithography that        utilizes programmable patterning structure, such as a        programmable mirror array of a type as referred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

The term “optics” as used herein and in the claims appended heretoindicates one or more optics and/or optical elements, such being anyobject capable of processing light in any known manner, such asreflection, refraction, transmission, diffraction, deflection,detection, and/or interference.

Before a pattern is projected onto a substrate, it may be desirable toaccurately position the substrate relative to a patterning structure, sothat the pattern will be projected onto the substrate accurately at arequired position. In modern semiconductor manufacturing, for example,it may be desirable or even necessary to realize such positioning withnanometer accuracy.

European patent application EP-A-1148390 describes such alignment, usinga phase grating alignment system. Such a system uses a substrate thatincludes an alignment structure having periodically variable opticalproperties. The phase grating alignment system measures the phase of theperiod of the alignment structure relative to some reference positionthat is defined (directly or indirectly) relative to the patterningstructure. From the phase, a measurement of the position of thesubstrate is obtained.

The phase measurement may be realized by incorporating the alignmentstructure in an optical interference arrangement that outputs light withan intensity that depends on the phase. The interference arrangement maycontain, for example, an imaging element that selects light diffractedby the alignment structure in a selected order of diffraction and imagesthe selected light onto a reference structure in the lithographicapparatus. This reference structure may have spatially periodicallyvariable optical properties, with a period that corresponds to theperiod of the image of the alignment structure. As a result, the lightoutput through the reference structure forms a kind of Moire pattern.This pattern varies as the image of the alignment structure movesrelative to the reference structure, dependent on the extent to whichthe image of the least reflective parts of the periods of the alignmentstructure coincide with the least transmissive parts of the periods ofthe reference structure. As a result, the spatially averaged intensityof the output light varies periodically as a function of the position ofthe substrate.

A similar periodic variation can also be realized without a referencestructure. European patent application No. EP 1148390 describes howinterference may be used between light from the alignment structure thatreaches a detector along two paths which correspond to images of thealignment structure that have been rotated 180 degrees with respect toone another. In this case, the centre of that rotation serves to providea defined position relative to the lithographic apparatus, and thedetected light intensity varies periodically as a function of theposition of the substrate relative to the defined position.

In such interference arrangements, the periodic variation of theaveraged intensity can be measured without requiring a detector withhigh spatial resolution. The measurement of the intensity of thedetected light makes it possible to assign phase values to differentpositions of the alignment structure. The phase values in turn may beused for accurate positioning of the substrate in the direction ofperiodical variation of the optical properties of the alignmentstructure.

Using a phase grating alignment subsystem allows a high alignmentaccuracy to be realized. Selection of an individual order of diffractionconsiderably reduces noise, since only light which has the spatialfrequency of the alignment structure is detected. Compared to alignmenttechniques that use electronic pattern recognition of images ofsemiconductor wafers, phase grating filtering techniques may realizeaccurate and low-noise alignment with detection and control circuitsthat are less critical for the alignment.

A phase grating alignment technique may have the problem that it onlyworks when sufficiently large alignment structures and/or referencestructures can be used such that the substrate can be initiallypositioned to image the alignment structure onto the referencestructure. In other words, such a technique may require the capturerange of the phase grating alignment to be sufficiently large. Largealignment structures and/or reference structures are expensive, e.g.because they occupy space on the substrate which could otherwise containcircuit components. Therefore, it is desirable to reduce the area-of thealignment structures and/or the reference structures. However, it hasbeen found that when an alignment structure is less than a given size,while the structure may still be suitable for accurate positioningpurposes, it may not always work immediately because the image of thealignment structure has no overlap with the reference structure when thesubstrate is initially positioned (i.e. insufficient capture range).

Also, the periodic alignment structure in combination with theinterference arrangement will provide an output signal with a certainperiodicity in it. E.g., when using a 8.0 μm phase grating as alignmentstructure and reference grating, the position can be accuratelypositioned, but only within 8.0 μm. When using a Nonius-principlemeasurement with an alignment structure having two different phasegratings of 8.0 and 8.8 μm, a periodicity of 88 μm exists. With theexisting (fine) alignment methods, therefore, there is a chance that anerror is made of one or more periods of the periodic signal. Here,periodicity is defined as the dimension of a single period of a periodicsignal.

An alignment subsystem may be included in a lithographic apparatus foraccurately measuring the position of substrate W to ensure thatsubstrate W is properly aligned during projection. FIG. 2 schematicallyshows an example 21 of an alignment subsystem that contains an opticalsubsystem with a radiation source 20, an imaging structure 24, referencestructures 26, 26 a (forming an optical interference arrangement),detectors 28, 28 a and a processing unit 29. Although processing unit 29is shown as one element, it will be understood that processing unit 29may be made up of a number of interconnected processors. Radiationsource 20 (for example, a laser) is arranged to generate a spot of lightthat is directed to an area 22 on substrate W. Imaging structure 24contains a lens arrangement 240, 242 to image area 22 onto referencestructure 26. In this example, reference structure 26 has spatiallyperiodic transmissive properties. Detector 28 is arranged to detect aspatially averaged intensity of radiation transmitted by referencestructure 26. Detector 28 has an output coupled to an input ofprocessing unit 29, which in turn has a control output coupled to secondpositioning structure PW, which is coupled to substrate W.

Interferometric measuring structure IF has an output coupled toprocessing unit 29. It will be understood that various changes may bemade to the alignment subsystem 21 without affecting its function. Forexample, mirrors may be added that may enable moving elements of thealignment subsystem to more convenient locations. In one embodiment, thealignment subsystem 21 is immediately next to the projection lens, butit will be understood that the alignment subsystem 21 may be furtherremoved from the projection lens. It is not necessary that the substrateis in the path of the projection beam during alignment. In fact, anothersubstrate (e.g. on a separate substrate table) may even be in the pathof the projection beam during alignment.

In operation, radiation from radiation source 20 is reflected from area22, and imaging structure 24 uses the reflected radiation to image area22 onto reference structure 26. The imaged radiation is partiallytransmitted by reference structure 26 onto detector 28, which generatesan electric or electronic signal that is indicative of the spatiallyaveraged intensity of the transmitted radiation.

Processing unit 29 uses this electric or electronic signal to generatecontrol signals for positioning structure PW. This operation involves anumber of stages (which may be executed by different elements (notshown) of processing unit 29). In a pre-positioning stage, processingunit 29 moves substrate W and/or the alignment subsystem 21 relative toone another so that an alignment structure is imaged onto referencestructure 26. In an accurate positioning stage, processing unit 29accurately measures the position of substrate W and the alignmentsubsystem 21 relative to one another (i.e. it determines for whichoutput value of interferometric measuring structure IF substrate W andalignment subsystem 21 are in a specific alignment relative to oneanother). Subsequently, processing unit 29 uses this measurement tocontrol one or more positions with a predetermined offset to a positionat which substrate W and the alignment subsystem 21 are in alignment, towhich substrate is moved for illumination with projection beam PB.

For accurate alignment, substrate W contains an alignment structure 10(see FIGS. 3, 5, 7, 9 and 10, to be discussed below) with spatiallyperiodic reflection properties in area 22. During accurate alignment,this alignment structure 10 is imaged onto reference structure 26. Thespatially averaged amount of light transmitted by reference structure 26depends periodically on the relative phase of the image of the alignmentstructure 10 and reference structure 26.

Preferably, imaging structure 24 passes only selected pairs of orders ofdiffraction onto reference structure 26. As shown, imaging structure 24has been designed to filter out selected orders of diffraction from area22. For this purpose, imaging element contains lenses 240, 242 with adiffraction order filter 244 in between. A first lens 240 maps lightdiffracted in respective directions to respective positions ondiffraction order filter 244, which transmits only light from selectedpositions. A second lens forms an image of area 22 from the transmittedlight. Thus only selected pairs of orders of diffraction are used forimaging onto reference structure 26. Without such selectivetransmission, position measurement is in principle also possible, but itmay have a worse signal-to-noise ratio.

It may be desired to treat separately a number of pairs of diffractionorders of the light from area 22. For this purpose, wedges 245 may beprovided to ensure that different pairs of orders are imaged ontodifferent reference structures 26, 26 a, each provided with its owndetector 28, 28 a. Although only two reference structures 26, 26 a andcorresponding detectors 28, 28 a have been shown (for pairs ofdiffraction orders ±1 and ±2, respectively), it should be understoodthat in practice a larger number of diffraction orders, for exampleseven pairs of diffraction orders ±n (n=1, 2, 3, 4, 5, 6, 7, . . . ) maybe treated separately, each with its own reference structure anddetector.

In an actual embodiment of the above described alignment subsystem 21,the imaging structure 24 may be arranged to filter out the 0-th order,effectively halving the period (or doubling the frequency) of thealignment structure 10. A 16 μm period of the alignment structure 10 onthe wafer W then effectively becomes a 8 μm period on the referencegrating 26.

Although single elements 240, 242 have been shown for the sake ofsimplicity, it should be understood that in practice the imagingstructure 24 may comprise a combination of lenses and/or imagingmirrors.

Furthermore, although a configuration has been shown wherein radiationis first reflected from substrate W and then transmitted throughreference structure 26 before detection, it should be appreciated thatother configurations may be used. For example, radiation reflected offreference structure 26 may be detected, and/or radiation transmittedthrough substrate W may be used if substrate W permits this. Similarly,radiation may be fed to reference structure 26 first (for reflection ortransmission) before being fed to substrate W prior to detection. Also,of course, the invention is not limited to a perpendicular incidence asshown in FIG. 2.

To allow accurate measurement of a wafer position (also called FineWafer Alignment or FIWA), the alignment structure or mark 10 in the area22 on the wafer W, in accordance with the prior art, may comprise twodifferent gratings for both the X- and Y-direction (see FIG. 3). Byusing two different grating periods for each of the marks 10 (e.g. 8.0and 8.8 μm) and for the corresponding reference structures 26, a muchmore accurate positioning and a larger capture range may be accomplished(using the so-called Nonius principle). The capture range is the rangein which an alignment can be executed without ambiguities. Because ofthe periodicity of the gratings, however, there is still an inherentambiguity in the fine positioning. In the case of the combination of the8.0 and 8.8 μm gratings, a periodic ambiguity of ±44 μm exists, whichcan lead to one or more 88 μm errors when the initial position of themark is outside the ±44 μm range. This effect means that the actualposition of the alignment mark 10 may be at a distance of n×88 μm fromthe detected position.

Coarse Wafer Alignment (COWA) followed by a FIWA may be performed incertain embodiments. In some cases, the COWA and FIWA may be executedsimultaneously. The FIWA may be performed using separate marks 10, suchas the known 8.0 μm phase grating, or the known combination of a 8.0 μmand 8.8 μm phase grating using e.g. the above described technique usingthe Nonius principle. Alternatively, the FIWA may be performed using themark 10 according to an embodiment of the present invention (when theseinclude at least a section with the normal period phase grating). Also,the present method and use of a phase grating as a mark 10 may beapplied to check whether the right initial position has been found toperform a FIWA using a single (e.g. 8.0 μm) grating mark (i.e. aconfidence check).

The coarse wafer alignment may be effected using a number of differenttypes of phase gratings for the mark 10. The underlying principle ofthese types is that a non-periodic feature of the phase grating is usedto avoid ambiguities that exist when only using the fine wafer alignmentmethod. Thereby, the capture range and/or the robustness of a periodicalignment system (e.g. using 8.0 μm gratings or a combination of 8.0 μmand 8.8 μm gratings) are enlarged beyond the periodicity of thatperiodic alignment system (8.0 μm and 88 μm, respectively).

The non-periodic feature may be included in the phase grating mark 10 onthe wafer in a number of manners. A number of marks according toembodiments of the present invention will be discussed below, togetherwith possible specific processing that may be desirable for suchembodiments. In general, two classes of marks 10 are disclosed, i.e. afirst class in which the phase of a detected signal is used to determinethe mark position (or better: non-periodic feature position), and asecond class in which the amplitude of a detected signal is used todetermine the mark position.

As a first type of mark 10, a so-called Phase Jump Mark is shown in FIG.3. In broken lines, the reference grating 26 (e.g. with a periodicity of8.0 μm) is shown, which is moved relative to the phase jump mark 10shown in non-broken lines. Such a phase grating mark 10 comprises linesand spaces as conventional alignment marks (e.g. spaced at 8.0 μm), butthe periodicity of the grating is broken at one or more positions 15 ofthe mark, resulting in a phase jump of the mark at this position(s). Thephase jump mark 10 may also be described as comprising two parts 11, 12with a phase grating having the same periodicity of lines and spaces,and a feature region 15 between the two parts 11, 12 where the space isnot filled with a phase grating having the same periodicity of lines andspaces as the two parts 11, 12. In a 8.0 μm phase grating, e.g., one ofthe spaces 15 may be reduced to 4 μm, 4/7 μm, or 200 nm or enlarged to12 μm (the 12 μm embodiment as shown in FIG. 3). With the 4 μm and 12 μmspaces, and the reference grating overlapping the two parts 11, 12, thephase difference between the reference grating and part 11 would differby π from the phase difference between the reference grating and part12.

Because of the non-periodic feature 15 in the periodic grating as shown,a phase change will occur when the reference grating 26 is moved withrespect to the phase grating 10 on the wafer W using the alignmentsubsystem 21 discussed above. This phase change is present in each ofthe detected orders n of the reflected light beam. The signal receivedon one or more of the detectors 28, 28 a may be processed to derive thephase of the received signal, e.g. by applying a best fit on themeasured and processed signal phases in a predetermined position window.At the point in the scanning direction (e.g. the x-direction) where thealignment signal phase changes at maximum speed, the captured alignmentmark position 15 is present.

Tests have been performed using a number of different phase changes (4μm, 4/7 μm, 12 μm and 200 nm), and the captured alignment mark 15 hasbeen detected using different orders of the diffraction grating. It hasbeen found that the 4 μm phase jump mark provides the best results withthe lowest order measurements (best reproducibility of results).

However, it has also been found that in using the phase jump marks 10with a rather large spacing (4 μm), the detected signal at the actualalignment position is actually the weakest signal over a larger positionwindow. This effect, of course, may result in unreliable results. It hasbeen found that the weak signal is the result of the left and right partof the phase jump mark 10 with a 4 μm spacing being out of phase. Whenthe spacing is made much smaller (in the order of 100 nm, e.g. 200 nm)this signal weakening effect may be much less prominent or even absent,and still the spacing 15 may be large enough to be detected using thephase detection method. In FIG. 4, such an effect is illustrated: FIG. 4a shows the alignment signal S as a function of the displacement x fromthe alignment subsystem 21 for the phase jump mark having a 4 μmspacing, and FIG. 4 b shows the alignment signal S as a function of thedisplacement x from the alignment subsystem 21 for the phase jump markhaving a 200 nm spacing. It can be seen that the alignment signal S forthe 200 nm spacing phase jump mark 10 has a larger signal strength atthe aligned position than the alignment signal S for the 4 μm spacingphase jump mark 10.

The non-periodic feature of the mark 10 can also be embodied as a linearphase profile mark (LPPM). One such embodiment is shown in FIG. 5. Inthis embodiment, two parts 11, 12 of the phase grating 10 both have aperiod (slightly) different from the reference grating 26. When movingthe reference grating 26 with respect to the LPPM 10, two linearlyvarying phase profile signals can be detected.

For one of the LPPM parts 11, 12 (taking the reference grating period of8.0 μm), the alignment signal then looks like $\begin{matrix}{{S(x)} = {{dc} + {A\quad{\cos\left\lbrack {\frac{2\pi}{\left( {8 + \Delta} \right)}x} \right\rbrack}}}} \\{= {{dc} + {A\quad{\cos\left\lbrack {\frac{2\pi}{8}\left( {x - {\frac{\Delta}{8}x}} \right)} \right\rbrack}\quad\left( {\Delta{\operatorname{<<}8}} \right)}}}\end{matrix}.$

It can be seen that the aligned position varies (Δ/8) micron per micronshift of the expected position. To have a non-periodic feature that mayallow detection of a unique position, the gratings 11, 12 of the LPPM 10are constructed such that the detected alignment phase signal Δ_(xap)shows two slopes with opposite slope sign (See FIG. 5 bottom part). Theunique alignment position can than be derived from the intersection ofthe two slopes.

In this embodiment, it should be kept in mind that the different periodsof the LPPM 10 may lead to different diffraction angles, which muststill be transmitted by the diffraction order filter 244 (see FIG. 2).The order positions are related to the mark period by$x_{ord} = {n_{ord}\frac{\lambda \cdot f}{d_{0}}}$where f is the focal distance and d₀ is the mark period. A change in theperiod of 8 nm will lead to a change in phase slope of 1 nm/μm, whichcan be accurately measured. A relative period change of 1%o will lead toa shift of the diffraction order of ˜3 μm.

The two slopes may be measured using a single reference grating 26 and asingle illumination spot or, as depicted in FIG. 5, using two referencegratings 26 and two illumination spots which both move in the directionindicated by the arrow to allow simultaneous determination of the twosloped signals.

In an even further embodiment of the first class of phase gratings 10,the variation of the grating period has a sinusoidal profile (SinusoidalPhase Profile Mark or SPPM). The two slopes in the resulting measuredsignal are then replaced by a sinusoidal shape, of which the top caneasily be detected. When the period of the sinusoidal phase profile ismuch larger than the period of the reference grating (e.g. >8 μm,preferably even >88 μm), a sinus fitting of the detected phase signalprofile would provide the capture position 15 of the alignment mark 10.Such a sinusoidal phase profile can be obtained from aposition-dependent period change:${\Delta(x)} = {\left. \frac{{\cos\left( {\frac{2\pi}{L}x} \right)} - 1}{x}\Rightarrow\quad{{phase}\quad{profile}} \right. = {{\cos\left( {\frac{2\pi}{L}x} \right)} - 1}}$where again we have the boundary condition that Δ(x)<<8. In thisequation, Δ(x) is the position-dependent period change, x is thedistance along the (longitudinal) x-direction, and L is the alignmentmark 10 length. An example of the sinusoidal phase profile (solid line),and the associated required position dependent period change Δ(x)(broken line) for the SPPM 10 is shown in FIG. 6.

The second class of marks 10 uses the intensity of the detectedalignment signal to provide the capture position of the non-periodicfeature 15 of the mark 10.

In a first embodiment of the second class of marks 10, use is made ofthe finite properties of regularly used marks, such as a versatilescribeline primary mark (VSPM), a regular 8.0 μm phase grating mark or acombination of a 8.0 μm and 8.8 μm phase mark. In an exemplaryembodiment, the marks present are scanned under an angle a to thelongitudinal direction of the mark 10. The angle α may vary between 0°and 90°, in which α=0° and α=90° are special cases. In FIG. 7, anexemplary arrangement of the scanning with the illumination spot and anexisting phase grating mark 10 comprising two separate phase gratings31, 32 is shown. In this case, both phase gratings 31, 32 present (e.g.an 8.0 μm and a 8.8 μm phase grating) are scanned by an illuminationspot 30 with the same (but opposite signed) scanning angle α. In FIG. 7,also a coordinate axis is shown, indicating that the phase gratings 31and 32 have their longest dimension in the x-direction and a smallerdimension in the y-axis.

The intensity signal detected by the alignment subsystem 21 for a singlescan (e.g. left part in FIG. 7, detecting the 8.0 μm grating 31) isshown in the plot of FIG. 8 as a function of the y-position. In FIG. 8,also the envelope of the detected signal is shown by the broken line,which allows a determination of the capture position. The detectedcapture position in this case, however, is the capture position in thedirection (y-direction) perpendicular to the longitudinal direction(x-direction) of the phase grating 31. When both phase gratings 31, 32are used as shown in FIG. 7, a more robust result of the captureposition may result.

The diagonal scan method as discussed with reference to the embodimentshown in FIG. 7, may also be executed using only a single grating 31.

The envelope of the intensity signal obtained from a phase grating 31,32 extending in the x-direction provides the capture position of thephase mark 10 in the y-direction. In a similar manner, the captureposition of a phase mark 10 extending in the y-direction will provide acapture position in the x-direction. When phase marks 10 are present onthe wafer in both the x- and y-direction in a known configuration (i.e.with known mutual offsets 35, 36, see the example shown in FIG. 9 withtwo phase gratings 31, 32 extending in the x-direction and two phasegratings 33, 34 extending in the y-direction), the found capturepositions in the x- and y-direction can be used as expected positions ina normal coarse alignment procedure (e.g. using a combination of 8.0 μmand 8.8 μm phase gratings as described above). Sometimes, it may stillbe desirable to perform a coarse alignment. For example, the accuracy ofthe diagonal scan procedure (the envelope signal), e.g. 30 μm dependenton the width of the phase grating marks 10 used, may be sufficient todetermine the correct top in a 88 μm periodic signal (as delivered bythe 8.0/8.8 μm phase. grating combination discussed above), but notsufficiently accurate to determine the correct top in a 8.0 μm periodicsignal.

Such a diagonal scan method may prevent making a periodic error, asduring a scan of a phase grating 31 . . . 34, when there will only beone distinct envelope shape present in the detected signal. When nosignal (or a signal below a predetermined threshold) is received at allusing the alignment subsystem 21, this result indicates that the phasegrating mark 10 has been missed. Then, the search window of thealignment subsystem 21 should be extended or shifted (e.g. with apositional shift equal to the (known) length of the phase grating 31 . .. 34). The diagonal scan method may be particularly suited to checkwhether the detected position of the wafer alignment, using a detectionmethod having possible periodic errors, is correct (confidence check).

For α=0°, the non-periodic feature 15 of the mark 10 is the start andend of the phase grating 31 . . . 34 in the longitudinal direction ofthe phase grating 31 . . . 34. The capture position (either the firstedge or the second edge of the phase grating 31 . . . 34, or acombination thereof) may be derived from the envelope of the detectedalignment signal, the length of which should generally correspond to thelength of the phase grating 31 . . . 34.

For α=90°, the non-periodic feature of the phase grating is the startand end of the phase grating 31 . . . 34 in the cross direction (i.e.perpendicular to the longitudinal direction). In this case, a captureposition in one direction (e.g. y-direction) may be determined using thex-direction phase grating 31, 32. In this case, when the referencegrating 26 and the phase grating mark 10 happen to be aligned so thatthey are exactly out of phase, it is possible that no signal is detectedat all using the alignment subsystem 21. This effect can also happenwhen the angle is very near to 90° and the lines are less wide than thespaces. However, a new scan with a very small change in the longitudinaldirection of the phase grating mark 10 will then provide sufficientsignal to determine the capture position in the y-direction. Thisspecial case of the diagonal scan may have a further disadvantagecompared to the diagonal scan with an angle 0°≦α<90°. For the case ofα=90°, the alignment subsystem may only detect the envelope signal andmay not detect an alignment signal in which also the periodic featuresof the phase grating 31 . . . 34 are present (which would allow thesignal also to be used for further (fine) wafer alignment).

The diagonal scan method of the alignment subsystem may provide a numberof further advantages, which may be predominantly present for the casesin which 0<α<90°. The scan doesn't necessarily use more time thanconventional scans, as the signal acquisition for capturing and aligningcan be performed simultaneously. Also, it is possible that no furtherhardware needs to be added to the alignment subsystem 21, as theilluminating beam (spot 30) and associated drive means, and thedetectors 28 and associated processing elements 29 may already bepresent. As the diagonal scan may use the same marks 10 on the wafer Was used for the fine wafer alignment, no further space may be needed inthe scribelanes on the wafer W. The robustness of the diagonal scanmethod may be the same as for the existing fine wafer alignment: when aphase grating mark 10 can be detected using the normal (x-direction)scanning of a phase grating mark 10, the phase grating mark 10 may alsobe detected using the diagonal scanning method. In present systems, thechoice of the size of the illuminating beam is a trade off: for optimalperformance it may be desirable for the beam to be very small to preventcross talk with other features on the wafer W. However, a very smallilluminating beam may enlarge the chance of missing the phase gratingmark 10 at all. Using the diagonal scan, however, the chance of actuallyhitting the phase grating mark 10 (which is much longer than wide) maybe greatly enhanced, allowing to actually use a small illuminating beam.

In a further embodiment of the present phase grating mark of the secondclass (intensity detection), the mark comprises a first part with aperiodicity of, in general, X μm (e.g. X=8.0 μm), and an adjacent secondpart with a periodicity of X/n, n being an integer number (e.g. n=7).This embodiment is shown schematically in FIG. 10 in which the mark 10comprises a first part 11 with a periodicity of 8 μm and a second part12 with a periodicity of 8/7 μm. When detecting the n-th orderdiffraction of this mark 10, signals with the same periodicity may bedetected over the entire mark 10, but there may be a noticeable changein intensity at the changeover 15 from X to X/n periodicity. This effectmay allow the detection of the capture position 15 of the mark 10. For arobust and effective error free detection of the capture position 15, itmay be desirable for the change in signal from the alignment subsystem21 to be as clear as possible, but still, it may be desirable for asignal to be detected from both parts 11, 12 of the mark 10. Inpractice, this effect implies that e.g. a combination of a 8 μm and 8/7μm (or 8/5 μm) phase grating will provide a workable mark 10 for coarsewafer alignment.

In an alternative version, not the periodicity is changed over the mark10, but the duty cycle of the spaces and lines is changed at one or morepositions in the phase grating 10. Duty cycle is the ratio of the widthof a line and a space in the alignment structure. Under normalcircumstances, phase gratings 10 are provided with lines and spaces ofequal dimension (duty cycle 50%). When the duty cycle of the lines andspaces is changed, this result may be noticeable in the amplitude of thesignal from the alignment subsystem 21 and may indicate the captureposition 15 of this embodiment of the mark 10.

FIG. 11 shows a flow chart of part of the operation of processing unit29. The flow-chart contains a prepositioning stage 301, an alignmentmeasurement stage 302 and a positioning-during-projection stage 303. Theprepositioning stage contains a number of steps. The steps of the flowchart are implemented, for example, by providing a suitable program ofinstructions in an instruction memory (not shown) of processing unit 29for execution by a conventional computer (not shown) in processing unit29.

Prepositioning stage 301 contains first, second and third tasks 304,305, 316. In first task 304 of the prepositioning stage, processing unit29 causes measurement of signals from detector 28 (and optionally fromdetector 28 a and further detectors) at a series of positions ofsubstrate W. For this purpose, first task 304 may include a firstsubtask 311 of moving the substrate or wafer W to a series of positionsrelative to the measuring subsystem and a second subtask 312 in which anintensity signal from detector 28 is measured at these positions. Therange of these positions is substantially larger than the size ofreference structure 26 (and the illumination beam 30). A third subtask313 ensures that the first and second subtasks are repeated fordifferent positions.

FIG. 12 shows a top view of part of a substrate W with an alignmentstructure 40. A scanning path 42 has been indicated. Path 42 is not partof substrate W, but indicates successive locations 44 of substrate W atwhich radiation source 20 provides spots of light on substrate W whichare imaged onto reference structure 26, during subtask 311. In anembodiment, the apparatus is arranged to scan along a meandering path oflines. This arrangement may provide for a two-dimensional search areawith minimal movement. Window 46 indicates the size of an area of pointson substrate W that are imaged onto reference structure 26 at a singletime. As can be seen, the image of alignment structure 40 may overlapwith reference structure 26 only for a limited number of positions alongscanning path 42 as indicated by the line (path 42) crossing thealignment structure 40.

In a second task 305 of prepositioning stage 301, processing unit 29computes amplitude information for a number of positions along scanningpath 42. For this purposes second task 305 may contain a first subtask314 in which the intensity amplitude information is computed. A secondsubtask 315 ensures that the first subtask is repeated for a number ofpositions. Although first task 304 and second task 305 have been shownseparately for the sake of clarity, it should be appreciated that bothsteps may be integrated, the amplitude information being computed as themeasurements come in. Such an arrangement may reduce the need for memoryfor storing measurements that are still needed for amplitudecomputations.

FIG. 13 shows an output signal of detector 28 as a function of position(in micrometers) along a horizontal part of scanning path 42 alongsubstrate W as a continuous curve (although, in practice, measurementsof the output may be performed only at certain sampling positions). Atpositions (around 500 micrometer) where the image of alignment structure40 overlaps reference structure 26, the output signal variesperiodically as a function of position. At other positions, a more orless random variation of smaller amplitude occurs, e.g. due tostructures present in substrate W that are not alignment structures(such as circuit structures, in the case of a semi-conductingmanufacturing process). When only selected pairs of orders ofdiffraction are used, the more or less random variations may involveamplitude and phase variations of a sine wave with a periodcorresponding to the selected orders of diffraction.

In the first subtask 314 of the second task 305, processing unit 29computes amplitude information of the variations of the output signalfor a number of positions (with coordinates x,y on substrate W asmeasured by interferometric measuring structure IF) along scanning path42. The computation of the amplitude information involves determining ameasure of the amplitude of the variations of the output signal. Forsubstrates W that do not produce much output signal in the absence ofthe alignment structure, it may suffice to detect the amplitude, forexample by taking the difference between minima and maxima (at differentpositions), or by measuring the maximum deviation from the averageoutput signal value. For other substrates W, a certain amount of spatialfiltering may be preferred to suppress the amplitudes of output signalvariations that do not have the spatial frequency expected for thealignment structure. This effect may be realized, for example, by bandpass filtering the output signal prior to amplitude measurement, or bycorrelation techniques that are selectively sensitive for the expectedoutput signal from alignment structure 40.

As one example of the computation in first subtask 313 of second task305, processing unit 29 computes the amplitude information according toA 2(x,y)=(Sum cos(k(x−x′))s(x′,y′))²+(Sum sin(k(x−x′))s(x′,y′))².

The sum is over locations with coordinates x′, y′, including at least aseries of locations with different x′ coordinate values spaced less thanthe width of an alignment structure from one another. In the example,A2(x,y) is the amplitude information A2 for a location with coordinatesx,y as determined from the output s(x′,y) of detector 28, with thesubstrate positioned at a locations with coordinates x′,y′. Theparameter k is equal to 2π/p, where p is the period of the image of thealignment structure 40 onto reference structure 26, 26 a. The range ofcoordinates x′,y′ extends over a window that extends around the positionwith the coordinates x,y for which the amplitude information iscomputed. The size of the window can be selected freely. A larger rangemay ensure larger frequency selectivity, making false detection ofalignment structures less likely, but may reduce position accuracy andvice versa. Preferably, the range of x′,y′ values that contributes tothe sums is taken to have approximately the same size as alignmentstructure 40.

In a fourth task 316 of prepositioning stage 301, processing unit 29uses the computed amplitude information to select a position (x0, y0)where the computed amplitude information is maximal as a function of aand y.

Although a formula for A2 has been given, by way of example it should beappreciated that without deviating from the invention other types offormula may be used. For example, a formula for the derivative of A2with respect to x may be computed to locate the maximum, or a productwith a weighting functions W(x−x′, x−y′) included in the sums, possiblywith W decreasing as x′ and/or y′ move away from x,y.

In principle, measurements from a detector 28 for a single pair ofdiffraction orders may suffice to find the maximum. Optionally, similardeterminations of amplitudes for different pairs of diffraction ordersmay be used, either to compute a combined amplitude function or toconfirm whether a maximum occurs for all pairs of orders of diffractionprior to using the location of the maximum.

Alignment measuring stage 302 is comparable to prepositioning stage 301,except that instead of an amplitude a phase is determined. Moreover, inalignment measuring stage 302, it may be desirable to make measurementsonly at positions where the alignment structure is at a position fromwhich it images onto reference structure 26.

In alignment measuring stage 302, processing unit 29 executes a firstsubtask 321 similar to first and second subtasks 311, 312 ofprepositioning stage 301, involving moving to a number of locations andmeasuring output signals of detector 28 for different positions ofsubstrate W. A second subtask 322 ensures repetition at a number ofdifferent locations relative to the alignment subsystem: These differentlocations have coordinates (x″, y0) that are selected dependent on theposition (x0, y0) produced in fourth task 316, with coordinates (x″, y0)wherein x″ is varied over a range around x0. In a third subtask 323,processing unit 29 computes a phase value φ from these measurements, forexample using a formula such astg(f)=a/b

-   -   where    -   a=Sum sin(k(x″−x0))s(x″,y″)    -   b=Sum cos(k(x″−x0))s(x″,y″)

The sum is over a range of locations with different x″. It may bedesirable for the different x″ values to extend over a range that isequal to the size of alignment structure 40, and/or it may be desirablefor the different x″ values to be spaced by less than a period ofreference structure 26. It is possibl that choice of the y″ values donot appreciably affect the computation, as long as they are within thesize of the alignment structure, and it may be desirable to use a rangeof y″ values in the sum. The computed phase value is proportional to thedisplacement between x0 and a reference position xr on substrate Wdivided by the period p with which alignment structure W is imaged ontoreference structure 26:f=2π(x0−xr)/p

Thus the phase value provides accurate position information of alignmentstructure 40 relative to the alignment sub-system. Inpositioning-during-projection stage 303 of the flow-chart, processingunit 29 uses this information to position substrate W at a series ofrequired positions relative to the patterning structure.

It may be desirable for the determination of the phase to be performedfor a number of pairs of orders of diffraction and/or for a combinationof the computed phases to be used to determine the position of thealignment structure.

It will be appreciated that different kinds of determinations of xr maybe used. For example, a weight function W(x″−x0,y″−y0) may be used inthe sums, or a recursive process may be used to adjust x0 until “b”becomes zero, in which xr may coincide with the adjusted x0.

It should be noted that the formulas for the phase φ and the amplitudeA2 may use the same type of elements: summing cosine functions and sinefunctions. Nevertheless, the formulas may lead to two different types ofinformation. The phase value φ may vary linearly substantially in thesame way as a function of position x0, no matter how wide the range ofpositions (x″,y″) over which the sums are taken. The amplitudeinformation A2, in contrast, may increase in strength as this rangeincreased, be it with an accompanying decrease in position dependence.This effect may make the amplitude information suitable for searchingamong areas that do not contain the alignment structure and/or the phaseinformation suitable for accurate positioning once it is known where thealignment structure 40 is located. The use of similar elements may alsomake it possible to use much of processing unit 29 for bothcomputations.

The determination of reference position xr from phase value φ may beambiguous, in the sense that any value of xr that differs by a integermultiple of the period p could be determined. Under some circumstances,the prepositioning accuracy may be sufficient to identify a single xrvalue as the correct value, but generally this is not the case.Therefore, it may be desirable to provide one or more additionalspatially periodic alignment structures on substrate W at predeterminedrelative positions with respect to alignment structure 40. The period orperiods of this additional alignments structure or structures may differfrom the period of alignment structure 40. Similarly, it may bedesirable to provide the alignment sub-system with additional referencestructures, detectors etc. to determine the phase of the intensityvariations due to displacement of the substrate relative to thealignment sub-system. Phase values determined in this way make itpossible to increase the distance between ambiguous xr values up to apoint that the prepositioning accuracy suffices to select the correct xrvalue. Alternatively, the additional alignment structures may bemeasured using the same reference structure. Such an arrangement mayallow prediction of the differences in aligned positions coming from thedifferent alignment structures based on a model and a priori informationon the shape and relative positions of the additional alignmentstructures and the first alignment structure.

In principle, the phase φ may only provide information about theposition of the alignment structure in a direction along which theoptical properties of alignment structure 40 vary periodically.Therefore, it may be desirable to provide one or more further alignmentstructures, reference structures etc. with periodical spatial variationin a direction transverse to the period of alignment structure 40 (e.g.perpendicular). It may be desirable to provide these one or more furtheralignment structures at predetermined positions relative to alignmentstructure 40. During alignment stage 302, these one or more furtheralignment structures may be used to obtain additional phase values foraccurately positioning substrate W in the direction transverse to thedirection perpendicular to the period of alignment structure 40.

It may be desirable for processing unit 29 to use the position (x0,y0)selected in third task 316 to control the positions of substrate W wherethe phase value of the additional alignment structure or structures andthe one or more further alignment structures is determined, usingamplitude information obtained by using at least one of the alignmentstructure, the additional alignment structures and the further alignmentstructures. Using a single one of the alignment structures forpre-positioning may simplify processing and is usually sufficientlyaccurate, both in a direction perpendicular to the period of thatalignment structure and in directions transverse thereto. But e.g. whenincreased pre-positioning accuracy is required, a combined amplitudevalue for different alignment structures may be used, for example bysumming amplitude information obtained for different alignmentstructures, taking account of the predetermined offset of thesestructures on substrate W.

FIG. 14 depicts a further top view of part of a substrate W. By way ofexample, substrate W may be a semi-conductor substrate on which areasfor different integrated circuit chips are reserved. Between the areasfor the integrated circuit chips, scribe-lanes 78 are provided which maybe sacrificed when substrate W is mechanically split into differentintegrated circuit chips. Periodic alignment structures 70, 72 arepresent in the scribe-lanes 78. For the sake of clarity, only twoperiodic alignment structures 70, 72 are shown, but it should beunderstood that more may generally be present. As shown in this figure,the optical properties of the alignment structures 70, 72 may varyperiodically along the direction in which scribe-lanes 78 run, i.e. inthe horizontal direction for horizontal scribe lanes between rows ofintegrated circuit chips and in the vertical direction for verticalscribe lanes between columns of integrated circuit chips.

In this embodiment, the extent of a periodic alignment structure 70, 72transverse to the direction of the scribe lane in which it isincorporated may be relatively narrow compared with its extent alongthat direction. Scanning paths 74, 76 run transverse to the direction inwhich the optical properties of periodic alignment structures 70, 72vary periodically.

In a method according to an embodiment of the invention, the amplitudeof the periodic variation of the intensity of the measured light for aparticular alignment structure is determined as a function of positionalong the scanning path 74, 76 transverse to the direction in which theoptical properties of periodic alignment structures 70, 72 varyperiodically (non-scan direction). To determine the amplitude of thevariation, it may be desirable for the intensity of the measurementlight for each position along the scanning path 74, 76 to be measured ata number of different positions in the direction in which the opticalproperties of periodic alignment structures 70, 72 vary periodically(scan direction). The position transverse to the direction of thescanning path may be determined where the amplitude shows best overlapof mark and reference structure. Typically, this is a position where amaximum amplitude occurs, but in case of a trapezoid shape of themaximum it may be possible for any position in a region with levelmaximum amplitude as a function of position to be used. This method maybe done for periodic alignment structures 70, 72 in two mutuallytransverse scribe lanes.

The results may be used to control respective components of the positionwhere the phase will be measured to realize accurate alignment. That is,the position of the maximum along the scanning path 74 transverse to thealignment structure 70 with optical properties that vary periodically inthe horizontal direction may be used to determine the vertical positioncomponent of alignment structure 72 for accurate alignment, and theposition of the maximum along the scanning path 76 transverse to thealignment structure 72 with optical properties that vary periodically inthe vertical direction may be used to determine the horizontal positioncomponent of alignment structure 72 for accurate alignment.

When the maxima are determined in scanning directions in which thealignment structures 70, 72 are relatively narrow, an accurate positioncan thus be determined. More generally, when the image of the alignmentstructure is narrow, an accurate position may be realized in this way (anarrow image may be a result of a narrow alignment structure, but alsoof imaging, for example of a form of filtering of orders of diffractionthat passes a wider bandwidth transverse to the direction of periodicvariation). Moreover, when the alignment structures generally extendover a relatively large extent transverse to the scanning path,pre-positioning is usually sufficiently accurate so that the apparatusmay need to try only one transverse scanning line for one alignmentstructure.

Only for small alignment structures, several parallel scanning paths mayneed to be -used, such that the scanning path where the highest maximumoccurs may be identified and used to determine the position.

To determine the amplitude for a position along a scanning path 74, 76,it may be desirable for a plurality of measurement intensities for eachposition along the scanning path 70, 72 to be measured at a number ofdifferent positions in the direction in which the optical properties ofperiodic alignment structures 70, 72 vary periodically. For example, ameandering scanning path may be used which extends far less in thedirection of periodic variation than in the transverse direction, or aseries of short scans along the direction of periodic variation at anumber of positions in the transverse directions.

However, alternatively only a single intensity measurement may be usedfor each position along the scanning path 74, 76, which in this caseruns perpendicular or under a non-zero angle to the direction ofperiodic variation. Thus, a intensity measurement for the same phase maybe obtained at each position along the scanning path 74, 76. As aresult, the relative maximum among these measurements may still becharacteristic for maximum amplitude. However, this approach may have arisk that an inconvenient phase is used. Therefore, it may be desirableto determine the maximum for at least two parallel scanning paths thatare a quarter of a period (plus any number of full periods) apart,although it may also be possible to use any other distance unequal to aninteger number of half periods.

As described, alignment may be performed in a number of distinct stagesand steps. It will be understood that a decomposition into stages andtasks may merely serve to facilitate explanation of the alignmentprocess. In practice, the various stages and steps need not be distinct.For example, the phase may be determined from stored intensitymeasurements that have been obtained before, after, or concurrently with(or are indeed part of) the measurements that are obtained at a seriesof relative positions of the substrate and the alignment measuringsystem for determining the position of maximum amplitude. In such case,the determination of the location of maximum amplitude may merely serveto select which of the stored intensity measurements should be used.Alternatively, separate intensity measurements performed at locationsselected under control of the location of maximum amplitude may be usedfor determining the phase. This approach may require less storage spacefor measurements, but it may increase the time needed for alignment,since substrate W may have to be moved to the selected locations.

Furthermore, although it may be desirable to use a suitably programmedcomputer or set of computers for the determination of phase andamplitude, as well as the search for the relative position of substrateand alignment measuring system that leads to maximum amplitude, it willbe understood that dedicated (not programmable, or only partlyprogrammable) elements may be used to perform any one or a combinationof these operations. For example, a digital signal processing circuitmay be used to perform the filter operations that filter out intensityvariations of the expected frequency and to compute the phase andamplitude. As another example, specialized hardware amplitude detectioncircuitry may be used, e.g. circuitry such as used for AM radiodemodulation may be coupled to detector 28.

FIG. 15 shows an alternative alignment system. Herein referencestructure 26 has been omitted. Instead a rotating element 60 has beenadded, which combines light diffracted in orders of diffraction withmutually opposite sign +n, −n rotating the image of the alignmentstructure of the opposite orders over 180 degrees relative to oneanother, e.g. rotating one over 90 degrees and the other over −90degrees. Such a structure has been described in European patentapplication No. EP 1148390. The intensity of the combined light ismeasured at a detector 28. In this case, it is possible that noreference structure is needed in front of detector 28. The output ofdetector 28 may be used in the way described for FIG. 2.

In a lithographic apparatus according to an embodiment of the invention,a alignment subsystem may be further arranged to use a non-periodicfeature provided on the alignment structure which is detectable as acapture position or a check position. A check position may e.g. be usedas a starting point for further alignment or as a confidence check.

By including a non-periodic feature in the alignment structure, whichcan be detected using the normal alignment subsystem, a precise capturepoint or position for starting a fine wafer alignment can be provided bysuch a lithography apparatus. Such a capacity may allow removal of aperiodic error ambiguity of existing (fine) wafer alignment methods, inany case beyond the periodicity of the fine alignment method.Alternatively, the lithographic apparatus may be used to checkafterwards (i.e. after or during alignment of the wafer) whether or nota periodic error has been made. For example, using the present alignmentsubsystem embodiment a priori, the capture range of the alignmentsubsystem may be extended, and using the alignment subsystem aposteriori as a check may increase the robustness of the alignmentsubsystem.

The non-periodic feature of the alignment structure may either induce adetectable phase shift in the detected signal, or a detectable intensityshift of the detected signal. Various embodiments exist which each mayprovide their own specific advantages.

In one embodiment of the phase shift inducing non-periodic feature, thenon-periodic feature is formed by a phase shift between two parts of thealignment structure by a change of the width of one (or more) of thelines or spaces of the alignment structure. In this case, the alignmentstructure has two parts with the same periodicity of lines and spaces,but one (or more) of the spaces is reduced or enlarged in length toprovide the non-periodic feature. One exemplary form reduces the spaceat the intersection of the two parts to half the periodicity of thephase grating (e.g. 4 μm in the case of a 8.0 μm phase grating) orextends it to one and half the periodicity (i.e. 12 μm for the samephase grating). Effectively, in the transition region, the contributionsto the measured signal of the alignment subsystem is then exactly inopposite phase for the two parts, which allows easy detection of thecapture position (i.e. the position where the phase gradient of thedetected signal is maximum).

However, due to the opposite contributions, problems may anfse due tothe resulting low amplitude of the detected signal. This effect may beprevented by making the space even smaller (e.g. 200 nm in the case of a8.0 μm phase grating), which results in a detected signal in which thephase change is still detectable, while the amplitude of the detectedsignal remains high.

In a further embodiment of the phase-shift inducing system, the opticalinterference arrangement comprises a reference grating, and thenon-periodic feature comprises a transition from a first part to asecond part of the alignment structure. The first part has a periodicitywhich is less than the periodicity of the reference grating, the secondpart has a periodicity which is greater than the periodicity of thereference grating, and the alignment subsystem is arranged to detect thecapture position or check position from the resulting sloped phaseinformation of the measurement light. Here, again, periodicity is thedimension of a single period in a periodical system, i.e. the dimensionof a combination of a line and a space (greater periodicity thus meansthat the distance between two consecutive lines is larger). By properlyselecting the periods of the first and second part of the alignmentstructure with respect to the reference phase grating, the phase of thedetected signal may be made to change linearly with varying mutualdisplacement, but with opposite sign for the first part and second part.The capture position or check position can then be derived from theintersection of the two sloped phase signals.

The resulting phase of the detected signal may in a further embodimenthave the form of a sinusoidal shape, which allows an easy detection ofthe capture position, e.g. using a sinus profile fitting of the.detected phase signal. For this, the alignment structure may comprise aposition-dependent period change according to${\Delta(x)} = \frac{{\cos\left( {\frac{2\pi}{L}x} \right)} - 1}{x}$in which Δ(x) is the position-dependent period change, x is the positionalong the alignment structure, and L is the length of the alignmentstructure over which the phase should vary, and the alignment subsystemis arranged to detect the capture position or check position from theresulting sinusoidal shaped phase information of the measurement light.

Advantageously, the sinusoidal phase profile mark may be designed suchthat the resulting sinus curve of the phase of the detected signal has aperiod which is much larger than the periodicity of the fine alignmentmethod (e.g. greater than 88 μm).

For the second class of marks, in which the intensity of the detectedalignment signal is used to determine the capture position or checkposition, a first embodiment utilizes the finite dimension of thealignment structure as a non-periodic feature. In this case, thealignment subsystem is arranged to detect the capture position from theenvelope of the intensity of the measurement light. In general,alignment structures in phase grating alignment systems are longer thanthey are wide, and the fine wafer alignment is executed using a scan ofthe alignment structure along its longest direction. To find a captureposition for the coarse wafer alignment, the envelope of the detectedalignment signal corresponds with the dimensions of the alignmentstructure used, and thus allows a capture position to be found.

In a further embodiment, the alignment structure has a first dimensionin a first direction and a second dimension in a second direction, inwhich the second direction is substantially perpendicular to the first,and in which the non-periodic feature is the first and/or seconddimension of the alignment structure. This first and/or second dimensionmay be detected using a scan of the alignment mark at an angle α between0° and 90° with the first direction.

This embodiment may be used for all kinds of alignment structures, andis also called diagonal scan. When the scan is performed at an angle ato the first direction, the capture position in the second direction maybe found. In general, alignment is performed in both the first andsecond direction, using mutually perpendicular alignment structures. Sowhen scanning a first alignment mark (extending in the first direction),a capture position may be derived for the second alignment mark(extending in the second direction), and vice versa.

In a further embodiment, the diagonal scan is performed at an angle αlarger than 0°. For example, it may be desirable to perform the scan atan angle α larger than 10°, e.g. at an angle α of 45°.

This embodiment may also be used when two alignment structures arepresent on the wafer for the alignment subsystem (e.g. a 8.0 μm and a8.8 μm phase grating). After a diagonal scan is performed at an angle αto the first direction for the first alignment structure, the scanningis repeated for the second alignment structure, but now at an angle −αto the first direction. This approach may provide a capture positionwith a higher confidence (more robust solution).

For α=0°, the scanning may be performed along the first direction only.Due to the finite dimension of the alignment structure, also in thefirst direction, this approach may provide a capture position in thefirst direction.

For α=90°, the scanning may be performed along the second direction ofthe alignment structure. As the dimension of the alignment structure inthe second direction is precisely known, this approach may allowdetermination of a capture position in the second direction, which maybe used when performing fine alignment using a second alignmentstructure. In this case, it is possible that the reference grating andthe alignment structure are precisely aligned, so that a very smallresulting signal is detected. This effect may be prevented e.g. by usinga reference grating with a periodicity different from the alignmentstructure, or by performing a new scan at a slightly offset startingposition.

In a still further embodiment, the non-periodic feature is formed by thetransition from a first part of the alignment structure to a second partof the alignment structure, the first part having a periodicity of X μmand the second part having a periodicity of X/n μm, n being an integernumber. The alignment subsystem is arranged to detect the captureposition or check position from a change in the intensity of an n-thorder of diffraction of the measurement light. Two-part alignmentstructures may be relatively easy to produce, especially when choosingthe parameters correctly. It may be desirable to take care that theshift in intensity of the detected alignment signal is large enough tobe detected reliably, but also that the lower signal still has asufficiently high amplitude. The invention could be implemented using acombination of a 8.0 μm part and a 8.0/7 μm part. Also the combinationof X=8.0 μm and n=5 (grating period of 8.0 and 1.6 μm, respectively) maybe implemented, as well as other suitable combinations.

In an alternative embodiment of a two-part alignment structure, thenon-periodic feature is formed by the transition from a first part ofthe alignment structure to a second part of the alignment structure, thefirst part having a first duty cycle value of the lines and spaces andthe second part having a second duty cycle value of the lines andspaces, and the alignment subsystem is arranged to detect the captureposition or check position from a change in the intensity of themeasurement light. Duty cycle is the ratio of the width of a line and aspace in the alignment structure. As a different duty cycle of a phasegrating in combination with the optical interference arrangement mayresult in a different amplitude of the detected signal, this approachmay allow detection of the capture position.

Further embodiments of the present invention provide for alignment ofthe substrate and the patterning structure in which a smaller alignmentstructure on the substrate suffices to provide for alignment andpre-positioning.

In a method according to an embodiment of the invention, alignment undercontrol of phase information involves a selection task, wherein aposition of the substrate is selected so that the alignment structureand the reference structure are in an overlapping imaging relationship.The selection task may involve determining the amplitude of intensityvariations of the measurement light output from the interferencearrangement and/or a search for a position of the substrate so that thealignment structure and the reference structure are in an overlappingimaging relationship. For such purpose, intensity measurements may besubjected to different processing for the alignment task and for thepre-positioning task, respectively. During alignment, the phase of theperiodic variations of the intensity is determined, and duringselection, amplitude information of the variations is determined,preferably after filtering out variations that do not match variationsdue to relative movement of the alignment structure. This operation isdone with a detector, which may be implemented as a suitablyprogrammable computer that processes the same type of intensityinformation that is also used for phase measurement, but specializeddetection hardware may be used as well. Subsequently, the alignment isperformed at a position at or near a maximum of the measure of theamplitude.

Although it may be desired is that radiation reflected from thealignment structure on the substrate is imaged onto the referencestructure, radiation transmitted through the reference structure beingdetected, it should be understood that the invention as claimed is notlimited to this embodiment. Instead of transmission though the referencestructure, reflection from the reference structure may be used andinstead of imaging radiation from the alignment structure onto thereference structure, one may use imaging the other way around from thereference structure onto the alignment structure on the substrate.

In some embodiments of the invention, the amplitude of the variations asa function of position of the substrate is filtered with a filter thatis selective for variations in the amplitude that correspond to thespatial frequency of the period of the alignment pattern and thefiltered amplitudes are use in the search. One way of implementing suchfiltering is to correlate the amplitude variations with a plurality ofmutually displaced versions of a basic matching pattern. Correlationtechniques are also often used to determine the phase, but in that casea single displaced version may suffice. By using correlation techniques,circuitry of a type that is used to determine the phase may also be usedto determine the amplitude information.

It may be desirable to use a phase grating alignment technique foralignment, wherein only selected orders of diffraction from thealignment structure are used to determine the amplitudes.

In some embodiments of the invention, the phase of the intensityvariation of the measurement light is used for alignment in onedirection, typically the direction in which the periods of the alignmentstructure follow one another. In such case, further alignment structureson the substrate and/or reference structures in the lithographicapparatus may be used to control alignment in another directiontransverse to said direction. Nevertheless, a single alignment structureand/or reference structure may be used for the two-dimensional searchfor a position of the substrate for the preposition prior to controldependent on the phase. Such an arrangement is possible because theamplitude of the intensity variations of the output radiation may beindicative of overlap in two dimensions even when the phase isindicative of position in only one dimension.

In some embodiments of the invention, only a single direction componentin a direction transverse to the direction of periodic variation of theperiodic alignment structure is determined from the amplitudemeasurements for an alignment structure. This approach may be done incases where the image of the alignment structures extends further in thedirection of the periodic variation than transverse to that direction.This effect may be a result of the fact that alignment structures areprovided in scribe-lanes on the wafers between adjacent chips. A largenumber of repetitions in an alignment structure may be realized alongthe length direction of a scribe-lane. Transverse to that direction, thealignment structure has a much narrower extent. This arrangement maylead to a better defined localization of the alignment structure as afunction of position along said transverse direction than along thelength direction of the scribe lanes. A similar effect may be caused bythe way the alignment structure is imaged onto the reference structure,for example by the shape of pupil apertures used to filter light fromselected order of diffraction. Therefore, in such an embodiment it maybe desirable to use the maximum only to search for the component of thealignment position along a direction transverse to the direction ofperiodic variation of the alignment structure.

Different alignment structures, with different directions of periodicvariation, may be present in mutually orthogonal scribe lanes areas on awafer. In some embodiments of the invention, the amplitudes obtainedfrom alignment structures with periodic variations in first and secondmutually transverse directions in different scribe lanes are used tosearch for different components of the alignment position transverse tothe first and second direction respectively. For example, alignmentstructures in horizontal scribe lanes may be used for searching for thevertical component of the alignment position and alignment structures invertical scribe lanes may be used for searching for the horizontalcomponent of the alignment position. Because an alignment structure mayextend over a large range along the length of the scribe-lane, theinitial position along this length is usually not critical during thesearch to ensure that the alignment structure is encountered when theapparatus scans transverse to this length direction.

In a device manufacturing method according to an embodiment of theinvention, the alignment structure comprises a non-periodic featurewhich is detectable as a capture position using the alignment subsystem.Device manufacturing methods according to this and further embodimentsof the invention may provide advantages which correspond to theadvantages mentioned above in relation to the associated apparatusand/or methods according to embodiments of the invention.

An alignment structure according to an embodiment of the inventioncomprises a non-periodic feature.

A lithographic apparatus according to an embodiment of the invention maycomprise a support structure for supporting patterning structure, thepatterning structure serving to pattern a projection beam of radiationaccording to a desired pattern; a substrate table for holding asubstrate with an alignment structure thereon, the alignment structurehaving spatially periodic optical properties; and an alignment subsystemfor aligning the substrate on the substrate table relative to thepatterning structure, the alignment subsystem comprising optics (e.g. anoptical arrangement) for optically processing light reflected from ortransmitted by the alignment structure, to produce measurement lightwhose intensity varies with the relative position of the spatiallyperiodic alignment structure and a reference position defined relativeto the patterning structure; with a sensor connected to the opticalinterference arrangement for measuring intensity and/or phaseinformation of the measurement light, and an actuator for controlling arelative position of the substrate table and the patterning structurebased on the intensity and/or phase information of the measurementlight.

A device manufacturing method according to a further embodiment of theinvention comprises providing a substrate that is at least partiallycovered by a layer of radiation-sensitive material, the substratecomprising an alignment structure with spatially varying opticalproperties; using patterning structure to endow a projection beam ofradiation with a pattern in its cross-section; aligning the substraterelative to the patterning structure, said aligning comprisingincorporating the substrate in an optical arrangement, which opticallyprocesses light reflected from or transmitted by the alignmentstructure, to produce measurement light of which the intensity varieswith the relative position of the spatially periodic alignment structureand a reference position defined relative to the patterning structure;measuring intensity and/or phase information of the measurement light;controlling a relative position of the substrate and the patterningstructure based on the intensity and/or phase information; andprojecting the patterned beam of radiation onto a target portion of thelayer of radiation-sensitive material.

An alignment structure according to an embodiment of the invention foraligning a work piece relative to a reference position using opticalmeasurements, such as interferometric measurements, comprises at leastone phase grating mark having a plurality of adjacent lines and spaceswith a predetermined periodicity.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention as claimed may be practicedotherwise than as described. It is explicitly noted that the descriptionof these embodiments is not intended to limit the invention as claimed.

1. An alignment structure comprising at least one phase grating markhaving a plurality of adjacent lines and spaces with a predeterminedperiodicity, wherein the alignment structure comprises a non-periodicfeature located between two parts of the alignment structure that havepredetermined periodicities along a line.
 2. The alignment structureaccording to claim 1, in which the non-periodic feature includes achange of the width of one of the lines or spaces of the alignmentstructure between the two parts of the alignment structure.
 3. Thealignment structure according to claim 1, wherein the non-periodicfeature comprises a transition from a first part of the alignmentstructure having a first periodicity to a second part of the alignmentstructure having a second periodicity, wherein the first periodicity isless than the second periodicity.
 4. The alignment structure accordingto claim 3, wherein the first periodicity is less than the periodicityof a reference grating, and wherein the second periodicity is greaterthan the periodicity of the reference grating.
 5. The alignmentstructure according to claim 1, wherein the alignment structurecomprises a position-dependent period change expressible as${\Delta(x)} = \frac{{\cos\left( {\frac{2\pi}{L}x} \right)} - 1}{x}$ inwhich Δ(x) denotes the position-dependent period change, x denotes aposition along the alignment structure, and L denotes a length of thealignment structure over which the phase varies.
 6. The alignmentstructure according to claim 1, in which the non-periodic featureincludes a transition from a first part of the alignment structure to asecond part of the alignment structure, the first part having aperiodicity of X microns and the second part having a periodicity of X/nmicrons, X being a positive nonzero number and n being a positivenonzero integer.
 7. The alignment structure according to claim 1, inwhich the non-periodic feature includes a the transition from a firstpart of the alignment structure to a second part of the alignmentstructure, the first part having a first duty cycle value of the linesand spaces and the second part having a second duty cycle value of thelines and spaces different than the first duty cycle value.
 8. Asubstrate including an alignment structure comprising at least one phasegrating mark having a plurality of adjacent lines and spaces with apredetermined periodicity, wherein the alignment structure comprises anon-periodic feature located between two parts of the alignmentstructure that have predetermined periodicities along a line.